Hybrid copper–silver nanoparticle inks and their performance under thermal and photonic sintering

Abstract

Silver (Ag) remains the dominant conductor for two-dimensional (2D) printed electronics applications. Whilst copper (Cu) is significantly cheaper and has comparable conductivity, it has found limited use to date due to its tendency to rapidly oxidise, which is detrimental to the electrical performance of printed features. Whilst oxidation may be avoided through ultra-fast photonic sintering, the need here for new and high capital expenditure slows down any transition from silver to copper. In this work, we investigate the addition of small amounts of Ag to Cu nanoparticle (NP) inks via a one-step continuous hydro/solvothermal process, with the aim of mitigating the effects of Cu oxidation under thermal sintering for the additive manufacture of functional Cu components. An in situ scanning electron microscopy (SEM) technique was used to visualise thermal sintering and the associated mass transport phenomena for Cu and Ag nanoparticles separately, in order to better understand the sintering behaviour of Cu–Ag composite films. The electrical performance of these thermally sintered hybrid films was compared with that of photonically sintered samples. In the case of intense pulsed light and laser-based techniques, sheet resistance values of ∼0.1 and ∼0.2 Ω sq⁻¹ were obtained respectively for Cu-only samples; the Cu NPs did not oxidise and thus Ag addition was superfluous. However, following thermal sintering in air, Cu-only NP samples were found to be oxidised and non-conductive. Cu NPs with 25 wt% Ag, sintered under the same conditions, demonstrated sheet resistance values of ∼1 Ω sq⁻¹. This improvement was attributed to Ag mobilisation and the subsequent formation of a continuous network of conductive pathways. Sheet resistance improvements were observed with as little as 3 wt%. Ag addition. This phenomenon was further investigated through theoretical simulations, in order to inform ink formulation and post-processing recommendations, to obtain the highest conductivity for the smallest Ag addition.